1. Field of the Invention
The present disclosure relates generally to semiconductor memory devices and, more particularly, to electrically erasable and programmable flash memory devices.
A claim of priority under 35 U.S.C. §119 is made to Korean Patent Application No. 2006-90867 filed on Sep. 19, 2006, the entire contents of which are hereby incorporated by reference.
2. Description of the Related Art
Semiconductor memory devices are generally divided into volatile memory devices and nonvolatile memory devices. Nonvolatile memory devices are those which can retain stored data even in the absence of power supply. On the other hand, volatile memory devices are those who lose stored data in the event of a power loss. Flash memories are one kind of nonvolatile memory. There are various types of flash memories such as, for example, programmable read-only memories (PROM), erasable and programmable read-only memories (EPROM), and electrically erasable and programmable read-only memories (EEPROM).
In addition to retaining stored data in the event of a power loss, flash memories are also more resistant to physical shocks and have faster access times for reading as compared to other memory devices. Because of these and other such features, flash memories are widely used as storage units in electronic apparatuses powered up by batteries. Furthermore, data is stored in a flash memory in memory blocks, each block including a plurality of memory cells. Generally, the memory cells are transistors which store data in the form of a voltage level. In addition, in single bit memory cells, each memory cell stores 1-bit data, but in multi-level memory cells, each memory cell includes at least 2-bit data.
In EEPROMs, programming and erasing operations are carried out by performing these operations in a plurality of memory blocks. To this end, generally, an EEPROM is erasable or programmable one memory block at a time. This means that EEPROMs may operate more rapidly and effectively during reading and writing operations by reading and writing data from and to multiple memory blocks in the EEPROM at the same time. The memory blocks in EEPROMs include insulation films. These films enclose charge storage elements. Charge storage elements are components used to retain data stored in the memory blocks. While these films may be used to protect the charge storage elements from undesirable electrical side-effects, these insulation films will wear out at some point as the number of reading and writing operations increase.
Thus, it is important to analyze the reliability of a flash memory device based on the available number of programming/erasing cycles (or endurance by operations), such that there is no degradation in data retention characteristics and data quality. There may be many other problems associated with the number of programming/erasing cycles in a flash memory device. For example, stored charges (or electrons) may easily leak out of the floating gate of a transistor because of various mechanical/electrical failures. These failures may include, for example, emission of thermions, charge diffusion, drifts of ionic impurities, stress by program disturbance, and so on. Furthermore, these failures are likely to decrease the threshold voltage of each memory cell in a memory block.
On the other hand, when the control gate of a transistor is held at a power source voltage, charges accumulate in the floating gate. This may lead to an increase in the threshold voltage of the memory cell. Furthermore, repetitive programming/erasing cycles may stress the oxide films of memory transistors and cause failures such as, for example, breakdowns of tunnel oxide films in the flash memory device. Such a failure may also lead to lower threshold voltages in memory cells. That is, under such conditions, charges would normally leak out of the floating gate of a programmed memory cell. As a result, as noticed by the broken line in FIG. 1, distribution profiles of programmed memory cells may move toward the lower voltages. Thus, there may be memory cells (those belonging to the shadow part) having threshold voltages lower than a program-verifying voltage. A program-verifying voltage is a voltage applied to a memory cell to ensure that the cell has been properly programmed. Because the threshold voltages of some memory cells may be lower than the program-verifying voltage, there may be errors in the program verifying operation. For example, if a memory cell has been programmed and has a program verify voltage of 5 v, if the threshold voltage of the cell drops below 5 v, a program verifying voltage applied to the cell may indicate that the memory cell has not been programmed although, in reality, the cell was programmed.
Thus, there is a need for structures and methods of programming flash memory devices which allow for repeated programming/erasing operations in the memory devices without causing as many electrical/mechanical failures as in conventional flash memory devices.
The present disclosure is directed towards overcoming one or more shortcomings associated with the conventional flash memory devices.